>OK, what did you do with pin#1, aka the BandWith pin, ?
>Leave it unconnected for less BW, but still more than enough for audio.

A 100 ohm resistor to the - rail. People claim it sound better this way. I only know it gets a bit warm

Regarding the stacking:
In the "Combining an amplifier with the buf634" application note they stack 3 buf634 with no seperate input or output resistors only one common ouput resistor (50 ohm).

I read somewhere that the buf634 has an output impedance of 10 ohm, so I though the an isolation resistor wasn't needed.

When I open my eyes and read the buf634 datasheet, it says: "Best response is usually achieved with a series input resistor of 25 to 200 ohm, depdending of the signal source. Response with some loads (especially capacitive) can be improved with a resistor of 10 to 150 ohm in series with the output".